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Phosphorus-Sulfur Heterocycles Incorporating an OP(S)

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Phosphorus-Sulfur Heterocycles Incorporating an OP(S) Article Phosphorus-Sulfur Heterocycles Incorporating an O-P(S)-O or O-P(S)-S-S-P(S)-O Scaffold: One-Pot Synthesis and Crystal Structure Study Guoxiong Hua, Kate Davidson, David B. Cordes, Junyi Du, Alexandra M. Z. Slawin and J. Derek Woollins * EaStCHEM School of Chemistry, University of St Andrews, Fife KY16 9ST, UK; [email protected] (G.H.); [email protected] (K.D.); [email protected] (D.B.C.); [email protected] (J.D.); [email protected] (A.M.Z.S.) * Correspondence: [email protected]; Tel./Fax: +44-1334-463861 Received: 7 September 2017; Accepted: 4 October 2017; Published: 10 October 2017 Abstract: A new one-pot preparative route was developed to synthesize novel organophosphorus- sulfur heterocycles via the reaction of the four-membered ring thionation reagent [2,4-diferrocenyl- 1,3,2,4-diathiadiphosphetane 2,4-disulfide (FcLR, a ferrocene analogue of Lawesson’s reagent)] and alkenyl/aryl-diols and I2 (or SOCl2) in the presence of triethylamine. Therefore, a series of five- to ten-membered heterocycles bearing an O-P(S)-O or an O-P(S)-S-S-P(S)-O linkage were synthesized. The synthesis features a novel application of the multicomponent reaction, providing an efficient and environmentally benign method for the preparation of the unusual phosphorus-sulfur heterocycles. Seven representative X-ray structures confirm the formation of these heterocycles. Keywords: 2,4-Diferrocenyl-1,3,2,4-diathiadiphosphetane 2,4-disulfide; diols; one-pot reaction; iodine oxidation; phosphorus-sulfur heterocycles 1. Introduction Organic disulfides have attracted considerable interest due to their practical applications in biochemistry, pharmacological chemistry, and industrial chemistry as well as their utility as synthetic reagents [1,2]. For instance, disulfides play an important role in the construction of secondary and tertiary polypeptide and protein structures. Furthermore, the formation of unsymmetrical disulfide bonds is required for the synthesis of many biologically active peptides, peptide mimetics [3], prodrugs, and antibiotics [4–6]. Disulfides have also been used for the preparation of self-assembled monolayers (SAMs) [7,8] and monolayer-protected clusters (MPCs) with many versatile properties [9,10]. The synthesis and reactivity of compounds containing multi-sulfur linkage have been studied extensively [11–17]. Many methods have been developed to prepare cyclic polysulfides by using either elemental sulfur or other sulfur-transfer reagents, however, these methods often gave poor yields of the desired products and require extreme precaution [18–20]. There are few samples for the synthesis of the sulfur-phosphorus compounds with O-(P)-S-S-(P)-O scaffold. The synthesis of alkyldithiophosphonate salts with O-(P)-S-S-(P)-O scaffold was first reported in 1970 [21]. Lawesson’s reagent reacted directly with ethylene glycol, 2,2-dimethyl-1,3-propanediol and 2,2′- dihydroxybiphenyl and tert-butylamine leading to di-tert-butylammonium salts, and the latter were further treated with I2 to give the corresponding cyclic disulfides [22]. Recently, three eight- membered phosphorus-sulfur heterocycles have been prepared via a one-pot reaction of alkenyl-diol with 2,4-diferrocenyl-1,3,2,4-diathiadiphosphetane 2,4-disulfide (FcLR, a ferrocene analogue of Lawesson’s reagent) [23]. The development of a highly efficient and systematic method for the preparation of phosphorus-sulfur heterocycles incorporating with an -O-P(S)-S-S-P(S)-O- scaffold is still desirable in organic synthesis. Herein, we report the synthesis and characterization of a series of Molecules 2017, 22, 1687; doi:10.3390/molecules22101687 www.mdpi.com/journal/molecules Molecules 2017, 22, 1687 2 of 14 new five- to ten-membered phosphorus-sulfur heterocycles with S-S linkage by treating 2,4- diferrocenyl-1,3,2,4-diathiadiphosphetane 2,4-disulfide (FcLR) with alkenyl-diols or aryl-diols in the presence of triethylamine followed an intramolecular oxidation by I2 or SOCl2, and seven related single crystal x-ray structures. 2. Results and Discussion 2.1. Synthesis and Characterization We have reported an efficient route for the synthesis of phosphorus-selenium macrocycles including a series of unusual nine- to fifteen-membered organoselenophosphorus macrocycles by means of Woollins’ reagent reacting with disodium alkenyl-diols, followed by in situ ring-closure reaction with appropriate dihalogenoalkanes [24,25]. A highly efficient route has also been developed successfully by our group for synthesis of a series of nevel twelve- to sixteen-membered organophosphorus-sulfur macrocycles via an one-pot three-component condensation of four- membered ring thionation reagent, 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane 2,4- disulfide (LR, Lawesson’s reagent) or 2,4-diferrocenyl-1,3,2,4-diathiadiphosphetane 2,4-disulfide (FcLR), alkenyl-dithiols or aryl-dithiols and dihaloalkanes in the presence of sodium hydride [26]. We adjusted the methods for the preparation of a series of phosphorus-sulfur heterocycles with the O-P(S)-S-S-P(S)-O scaffold. The preparation and spectroscopic characterization of FcLR have been reported by our group [27]. The one-pot reaction of FcLR with an equimolar amount of alkenyl-diol in the presence of two equimolar amounts of triethylamine at room temperature for 24 h, followed by addition of iodine led to the corresponding eight- to ten-membered phosphorus-sulfur heterocycles 1–6 in good to excellent yields (63% to 89%), respectively, as depicted in Scheme 1. Insoluble (in water or normal organic solvent) side products which we were not able to identify, but which we assume are linear polymers, were found resulting in the incomplete transformation in all cases. The results show that the reactions tolerate diverse alkenyl-diols with the non-branched or the highly-branched chains, however, the non-branched diols were found to be preferable for the formation of the products in high yields in the current heterocyclization in general, for instance, ethylene glycol gave the product 1 in the highest yield (89%) and the lowest yield (63%) was found in the formation of product 4 in where the butane-2,3-diol was used as starting material. We speculate that 1–6 was preferentially formed by intramolecular oxidation/cyclization of intermediate A in the presence of two equivalents of triethylamine. Scheme 1. Synthesis of eight- to ten-membered heterocycles 1–6 from FcLR, alkenyl-diols and I2. Molecules 2017, 22, 1687 3 of 14 Similarly, the reaction of 2,4-diferrocenyl-1,3,2,4-diathiadiphosphetane 2,4-disulfide [FcP(μ-S)S]2 (FcLR) with an equimolar amount of aryl-diol in the presence of two equimolar amounts of triethylamine at room temperature for 24 h, followed by the oxidation using iodine, gave rise to the additional mono-phosphorus species 7, 9 and 11 in 24%, 29% and 41% yields respectively together with the expected phosphorus-sulfur heterocycles 8, 10 and 12 in moderate yields (36 to 49%) as shown in Scheme 2. Once again, in all the cases, insoluble (in water or normal organic solvent) by- products which we were not able to identify were found. The reactions are satisfactory with aryl- diols. The results show that the multiple aromatic ring substitute was preferable to achieve high yields for the formation of both mono-phosphorus species and di-phosphorus heterocycle, for example, when [1,1′-biphenyl]-2,2′-diol is used as diol, the products 11 and 12 were obtained in the highest yields, compared to the other cases. We presume that 7, 9 and 11 were selectively formed by intramolecular cyclization of intermediate B by loss of a molecule of FcP(=S)(SHNEt3), and 8, 10 and 12 were preferentially formed in the same way as compounds 1–6. Scheme 2. Synthesis of mono-phosphorus species 7, 9, 11 and di-phosphorus species 8, 10, 12 from FcLR, aryl-diols and I2. Interestingly, the acyclic product 13 was obtained in 77% yield rather than the expected ten- membered heterocycle D when but-2-yne-1,4-diol was used as the starting material under similar reaction conditions (Scheme 3). The preparation and single crystal structure of the disulfide 13 has been reported previously [28]; the compound was synthesized by the iodine oxidation of sodium methoxy(ferrocenyl)dithiophosphonate salt, which was obtained from the reaction of Ferrocenyl Lawesson’s reagent with sodium metal and methanol. However, in that paper, there are no details of characterization and only two diastereomers were found. Here, four diastereomers (ca. 2:1:1:2 intensity ratio) were found in this product due to the two chiral centers present within the molecule. We propose that the heterocyclic compound D is formed via an oxidation/cyclisation of the intermediate C, however, the heterocycle D is not stable and readily decomposed to 13 by the loss of a molecule of IC≡CI in the presence of the oxidant I2, further investigation of the mechanism is on the way. Molecules 2017, 22, 1687 4 of 14 Fe i, HO OH/Et3N S Fe S S O O CH CN, r.t., overnight P P 3 P S P S ii, I ,CHCN,r.t.,2h S S 2 3 S Fe Fe FcLR 13, 77% OH OH/Et3N I I I2 I Fe S 2 Fe S O O O O P P P P Et3NH S 2 HNEt3 S HNEt3 S S S Fe S Fe C D Scheme 3. Synthesis of non-heterocycle 13 from FcLR, but-2-yne-1,4-diol and I2. Surprisingly, attempts to insert a heteroatom into the heterocyclic ring to create a S-S(O)-S linkage failed, and instead the disulfides 1–3 and 6 were obtained from the reaction of FcLR with alkenyl-diols, followed by the addition of SOCl2 at 0 °C (Scheme 4).
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